The impact of the composition and solidification rate on the microstructure and the crystallographic orientations of Al2O3-YAG-ZrO2 eutectic solidified by the micro-pulling down technique.

Abstract

Directional solidification experiments have been carried out in eutectic and off eutectic composition in Al2O3–YAG–ZrO2 system using the micro-pulling down (μ-PD) technique. The formation of different microstructures of stable phases, over a range of velocities studied, is discussed. During the solidification process, coupled growth microsctrucure can occur at off eutectic composition. Colonies and dendrites were analysed as a function of the composition and velocity. Increasing the solidification rate resulted in a smaller grain size and instability of the crystallization interface with respect to ZrO2 segregation at the solidification front. Increasing ZrO2 in the eutectic composition showed enhanced tendency for colony and dendrite formation. When the velocity was extended to the rapid solidification regime (>1.75 mm min−1), the geometrical pattern microstructure was found to grow and a change in the growth orientation of Al2O3 corresponding to c-axis (0 0 01) was observed. Whatever the pulling rate, during the eutectic solidification the Al2O3 and ZrO2:Y phases' growth competition was observed, and 〈100〉 YAG orientation was grown along the pulling direction.